For the azimuth measurement device for detecting the geomagnetism to calculate the azimuth, it is important to discriminate a geomagnetism (i.e., a steady magnetic field) and such a magnetic field other than the geomagnetism as is generated by a magnet or the like fixed in the azimuth measurement device. The magnetic field other than the geomagnetism is calculated as an offset (or a reference point) from a measured magnetic data group by proper means. The geomagnetism can be determined by subtracting that offset from the measured magnetic value.
This offset highly changes in dependence upon whether a magnetic element (e.g., a memory card) accompanying the azimuth measurement device is mounted or demounted. Alternatively, the user of the azimuth measurement device may evaluate that the calculated value of the offset is improper, and may require a recalculation intentionally.
As an azimuth measurement device of the prior art, an azimuth measurement device has been proposed in Patent Document 1. This azimuth measurement device measures the azimuth on the basis of the geomagnetism which is detected by using a Hall element. A corrected value storage section is stored with the reference values of an X-axis Hall element and a Y-axis Hall element. By using the reference values, a correction calculation section corrects the amplified output values of the X-axis Hall element and the Y-axis Hall element, and extracts only the values proportional to the individual axis components of the geomagnetism.
On the other hand, Patent Document 2 provides an azimuth measurement device, which is made highly accurate by correcting the error of the mounting angle between the direction of the input axis of the geomagnetism sensor and the direction of the reference axis of a position detection device. This azimuth measurement device comprises: a mounting angle correction operation section for performing a mounting angle error correction for geomagnetism data to operate the geomagnetism data corrected; a coordinate transformation circuit for transforming the coordinates of the geomagnetism data by using position data to output the geomagnetism data; and an azimuth calculation circuit for calculating the azimuth on the basis of the geomagnetism data.
Moreover, Patent Document 3 describes an azimuth measurement device using a geomagnetism sensor. The horizontal component of the geomagnetism is so detected that it is divided into voltage output signals of two components by magnetic detection coils orthogonal to each other. These voltage output signals are input to a microcomputer having signal correction operation means to operate azimuth data to the magnetic north of the geomagnetism. The azimuth measurement device is provided with correction processing means for operating corrected values corresponding to the individual azimuth data from the microcomputer.
On the other hand, azimuth measurement devices described in Patent Document 4, Patent Document 5 and Patent Document 6 are also known in addition to the aforementioned publications.
A device of the prior art of Patent Document 7 corrects the offset of the magnetic sensor in the azimuth measurement device and is enabled to calculate offset information with respect to the individual axis outputs of a magnetic sensor merely by changing the direction of a mobile telephone arbitrarily. Moreover, the calibration work of the offset is facilitated to lighten the burden on the user at the time when the offset is calibrated.
This application calculates the dispersion of reference points estimated, and discards the estimated reference point in case the dispersion is at a predetermined value or higher. If this predetermined value (as will be called the “acceptance threshold”) is set high, an offset of a large error may be calculated. Since the offset is easily updated, it is instantly corrected even if highly changed.
In case the decided value is small, on the contrary, the offset is hard to update. Incase, therefore, an offset of a small error has already been calculated, an estimated offset value of a larger error can be kept highly accurate by discarding it. In case, however, the offset highly changes, a long time is taken till the correct offset is adopted. For these reasons, a plurality of specifications such as the response rate or accuracy required for the azimuth measurement device are hard to satisfy at the same time, if the adoption and discard of the reference point are determined by using one acceptance threshold at all times.
The present invention has been conceived in view of those problems, and has an object to provide an azimuth measurement device and an azimuth measurement method for calculating a highly accurate offset quickly according to the situations from a magnetic data group acquired for measuring the azimuth.
Patent Document 1: Japanese Patent Application Laid-open No. 2003-065791
Patent Document 2: Japanese Patent Application Laid-open No. 2003-042766
Patent Document 3: Japanese Patent Application Laid-open No. 10-132568 (1998)
Patent Document 4: Japanese Patent Application Laid-open No. 61-147104 (1986)
Patent Document 5: Japanese Patent Application Laid-open No. 62-255814 (1987)
Patent Document 6: Japanese Patent Application Laid-open No. 8-105745 (1996)
Patent Document 7: Japanese Patent Application Laid-open No. 2003-035010